A. Field Of The Invention
The present invention is directed to the field of double-sided or multi-layer circuit boards. In particular, the present invention is directed to a composition and a process for preparing the non-conductive surfaces in the through holes of a multi-layer or double-sided circuit board for electroplating, wherein the electroplated through hole is of uniform diameter and substantially void free. The present invention is useful because it eliminates one of the defects commonly associated with the electroplating of through holes in a multi-layer or double-sided circuit board in both additive and semi-additive processes.
B. Background
Printed circuit boards refer to solid circuits that are formed from a conductive material that is positioned on opposite sides of an insulating material. Where the printed circuit board has two conductive surfaces positioned on opposite sides of a single insulating layer, the resulting circuit board is known as a "double sided circuit board." In order to make electrical connections between the circuits on opposite sides of the double-sided circuit board, a hole is first drilled through the double-sided circuit board i.e., through the two conducting sheets and the insulator board. These holes are known in the art as "through holes." To accommodate even more circuits on a single board, several copper layers are sandwiched between boards of insulating material to produce a multi-layer circuit board. Like the double-sided circuit boards, the multi-layer circuit boards also use "through holes" to complete circuits between the circuit patterns.
Various processes have evolved over the years for forming a conductive pathway between the various circuits, via the through hole. Initially, a number of mechanical means (e.g., rivets or eyelets) were used. However, these were labor intensive to install and proved unreliable with age.
U.S. Pat. No. 3,099,605 (Radovsky), which issued on Jul. 30, 1963, discloses an improved method for forming a conductive pathway through a through hole by first coating the through hole with colloidal palladium (PdCl.sub.2 +SnCl.sub.2) and then electroplating over the coating. Radovsky states that prior efforts, which used graphite to form a conductive base coating on the exposed areas of a through hole, suffered from many "defects." ['605 at col. 1, ln. 66]. These defects were said to include the "lack of control of the graphite application with the resultant poor deposit of the electroplated metal and non-uniform through hole diameters." ['605 at col. 1, ln. 66-70].
Radovsky also teaches the recognition in the art that the electroless plating method (i.e., chemical depositing) "has advantages over the graphite methods." [Id. at col. 2, ln. 10-12]. "[The] advantages are essentially better control over the base layer of catalyst metal deposition and a resultant improved electroplating process with more uniform hole diameters." [Id. at col. 2, ln. 12-15].
It is an object of the present invention to provide a composition that uniformly coats the non-conductive surface(s) of a through hole and that allows a uniform deposit of electroplated metal to securely form thereon.
U.S. Pat. No. 3,163,588 (Shortt), which issued on Dec. 29, 1964, briefly suggests that a through hole surface may be rendered conductive prior to electroplating by either chemical ("electroless") deposition of a metal or by applying a "paint or ink" for instance, containing a substance such as "graphite." ['588 at col. 3, ln. 57-58]. However, the above description of Radovsky, which was filed on Dec. 30. 1959, more than three years after the filing date (Feb. 14, 1955) of Shortt, describes the "defects" associated with using a paint or ink containing graphite, such as referenced in Shortt. Because of industry recognized defects, such as pointed out in Radovsky, the industry turned to electroless deposition.
U.S. Pat. No. 4,619,741 (Minten), which issued on Nov. 11, 1986, teaches that for a "quarter century" prior to its filing date, (May 5, 1986), the industry relied upon the "electroless copper deposition" to prepare the walls of a through hole for electroplating. ['741 at col. 1, ln. 25-28]. Although electroless deposition provided superior results to the prior art methods for preparing a through hole surface, electroless deposition has several commercial disadvantages. As pointed out by Minten, these disadvantages include a six step process prior to electroplating; "a reactively long process time;" "multiple treatment baths" ; a "complex chemistry which may require constant monitoring and individual ingredients which may require separate replenishment;" a "palladium/tin activator [which] also may require extensive waste treatment;" and "the multiplicity of rinse baths [which] may require large amounts of water." ['741 at col. 1, ln. 66 to col. 2, ln. 7].
To overcome the stated disadvantages associated with the electroless deposition method, the '741 patent coats the non-conductive surface(s) of a through hole wall of a printed circuit board with carbon black particles prior to electroplating. The '741 patent expressly teaches that "graphite particles" are not capable of substituting for the carbon black particles: "when graphite particles are used as a replacement for the carbon black particles of this invention, loss of adhesion of the copper to the non-conducting material after the subsequent electroplating was noted. See comparisons 1 and 2 below." ['741 at col. 7, ln. 11-16]. In comparison I of the '741 patent, through holes that were electroplated after application of the first substitute graphite formulation (2.5% by weight graphite) had only a few visible voids, "but failed the solder shock test." ['741 at col. 20. ln. 5-7]. According to the '741 patent, "[t]he plated on copper in the holes pulled away from the epoxy/glass fiber layer." [Id. at ln. 7-8]. The results were even worse with the second substitute graphite formulation (0.5% by weight graphite). After electroplating, the boards that were treated with the second substitute formulation "had no or very few unvoided holes" (i.e., they had voided holes) ['741 at col. 20, ln. 14]. According to the '741 patent, "[t]he standard shock test could not be run on boards that were prepared with this latter graphite formulation because of the lack of unvoided holes." ['741 at col. 20, ln. 14 -16]. According to the '741 patent, "both graphite formulations were far inferior for electroplating preparation as compared to the above carbon black formulations." ['741 at col. 20, ln. 17-19.
Similarly, the following U.S. patents teach that graphite is not a substitute for carbon black in carbon black formulations that conductively coat through holes prior to electroplating: U.S. Pat. No. 4,622,108 (Polakovic) at col. 8, ln. 1-5; 4,631,117 (Minten) at col. 7, ln. 24-28 ("when graphite particles are used as a replacement for the carbon black particles of this invention, the undesirable plating characteristics mentioned in U.S. Pat. No. 3,099,608 would likely occur." ); 4,718,993 (Cupta) at col. 8, ln. 27-37; and 4,874,477 (Pendleton) at col. 7, ln. 60-68.
In addition, the following U.S. patents discuss relative to the prior art the deficiencies associated with using graphite as a conductive coating prior to electroplating: U.S. Pat. Nos. 4,619,741 at col. 2, ln. 16-25; 4,622,108 at col. 2, ln. 12-20; 4,622,107 at col. 1, ln. 52-60; 4,631,117 at col. 2, ln 22-30; 4,718,993 at col. 2, ln. 21-29; 4,874,477 at col. 1, ln. 54-62; 4,897,164 at col. 1, ln. 54-62; 4,964,959 at col. 1, ln. 28-36; 5,015,339 at col. 1, ln. 56-64; 5,106,537 at col. 1, ln. 34-42; and 5,110.355 at col. 1, ln. 60-68. According to these patents, the deficiencies with the graphite process included lack of control of the graphite application, poor deposit of the resultant electroplated metal, non-uniform through hole diameters, and low electrical resistance of the graphite.
It is an object of the present invention to develop a composition that is capable of depositing a controlled and a uniform coating of graphite particles on the non-conductive surfaces of a through hole. It is also an object of the present invention that the uniformly deposited graphite coating be capable of eliminating the need for electroless plating prior to electroplating. It is a further object of the present invention that a metal electroplated over the deposited graphite coating be capable of withstanding the solder shock test.